Presently, various methods are developed for fabricating the fiber-optic filter. For example, in the case of U.S. Pat. No. 6,636,684B1, which is entitled “Dispersive Optical Fiber Using Binary Component Slica”, cores and claddings of fibers with different material dispersions are utilized to fabricate a fiber-optic filter. Besides, in the case of U.S. Pat. No. 5,673,342, which is entitled “Communication System Comprising A Low Cost Optical Filter”, a core and a depressed cladding of a fiber is utilized to fabricate a waveguide dispersion. However, these methods both need specialized kinds of fibers, which cost much and have high optical losses.
Regarding the academic research, moreover, many efforts are also done for improving the fabrication of the fiber-optic filter. For example, in the disclosure of K. McCallion, W. Johnstone, and G. Fawcett, “Tunable in-line fiber-optic bandpass filter,” Opt. Lett. 19, 542–544 (1994)”, an optic film which is positioned on the side-polished fiber is utilized to fabricate a fiber-optic band-pass filter. Besides, in the disclosure of G. Raizada and B. P. Pal, “Refractometers and tunable components based on side-polished fibers with multimode overlay waveguides: role of the superstrate,” Opt. Lett. 21, 399–401 (1996)”, a similar method is also utilized to fabricate a fiber-liquid-refractive-index meter. The fiber-optic band-pass filter as well as the fiber-liquid-refractive-index meter fabricated by the mentioned methods, however, both have a poor accuracy and need to be further improved.
In the disclosure of M. Tammer, R. W. T. Higgins, and A. P. Monkman, “High optical anisotropy in thin film of polyfluorene and its affect on the outcoupling of light in typical polymer light emitting diode structures,” J. Appl. Phys. 91, 4010–4013 (2002)”, a conjugated polymer having a high dispersive characteristic around a light-high-absorbed location is disclosed. A polymer with a sharper slope of the refractive index dispersive curve than that of the fiber is able to be utilized to fabricate a long-wavelength-pass filter. On the contrary, a polymer with a flatter slope of the refractive index dispersive curve than that of the fiber is able to be utilized to fabricate a short-wavelength-pass filter. Besides, the long-wavelength-pass filter and the short-wavelength-pass filter can be concatenated to fabricate a band-pass filter. Certainly, other polymers with a specialized shape of dispersive curves are also utilized to fabricate a band-pass filter or a band-rejection or notch filter.
In the disclosure of H. R. Stuart, “Dispersive multiplexing in multimode optical fiber,” Science 289, 281–283 (2000)”, several spatial modes of a multi-modes fiber is developed for being used in the application of signal multiplexing. Hence, a fiber-mode filter is achievable by the elements with multi-modes fibers, since the effective refractive indices of different transmission modes thereof fibers are respectively different.
In the disclosure of Y. Miyamori, Y. Ashi, E. Sato, and M. Takano, “PON-based all fiber-optic access system,” Hitachi Review 47, 63–66 (1998)”, it is mentioned that the traditional bi-directional techniques (1310/1550-nm) is replaced by the Tri-band techniques (1310/1490/1550-nm) in the WDM-PON network with FTTH systems in Japan.
Besides, in the attached several disclosures, the aforementioned dispersive fibers and the applications thereof for fabricating filters are also mentioned. However, such schemes are disadvantageous in the high costs because of the adoption of the specialized fibers, and hence the schemes are hard to be popularized.
The following disclosures are cited as the references for the present invention.    [1] J. W. Yu and K. Oh, “New in-line fiber band pass filters using high silica dispersive optical fibres,” Opt. Commun. 204, 111–118 (2002).    [2] K. Morishita, “Optical fiber devices using dispersive materials,” J. Lightwave Technol. 7, 198–201 (1989).    [3] K. Morishita, “Bandpass and band-rejection filters using dispersive fibers,” J. Lightwave Technol. 7, 816–819 (1989).    [4] K. Morishita, M. S. Yataki, and W. A. Gambling, “In-line optical fibre filters using dispersive materials,” Electron. Lett. 23, 319–321 (1987).    [5] J. Nishimura, Y. Ueda, and K. Morishita, “Fabrication of dispersive fibers and their application to long wavelength-pass filters,” Electronics and Communications in Japan, 79, 9–15 (1996).    [6] K. Morishita and S. Yutani, “Wavelength-insensitive couplers made of annealed dispersive fibers,” J. Lightwave Technol. 17, 2356–2360 (1999).    [7] J. Nishimura and K. Morishita, “Mode-field expansion and reduction in dispersive fibers by local heat treatments,” J. Select. Topics Quantum Electron. 5, 1260–1265 (1999).